This Program Project brings the cumulative expertise and varied intellectual perspectives of 14 established investigators in the Eaton- Peabody Laboratory to bear on a number of fundamental issues underlying vertebrate auditory mechanisms. The laboratory offers broad-based coverage of scientific disciplines, including anatomy, physiology, chemistry, pharmacology, electrical engineering, and psychology, coupled with clinical expertise in neurology and otolaryngology. Nine proposed projects target a number of areas that are still poorly understood. Middle-ear studies combine the perspectives of engineering-based researchers of middle-ear mechanics with the surgical perspectives of practicing otologists to tackle issues concerning non-ossicular acoustic transmission, issues directly relevant to surgical management of middle-ear pathologies. Inner-ear studies combine physiological and electrical-engineering expertise to investigate mechanisms underlying ionic homeostasis in the sensory epithelium, an understanding of which is key to explaining the cellular response to a wide range of insults to the inner ear, including noise- or drug-induced hearing losses. In an interconnected grouping of four anatomical, physiological and pharmacological projects, seven researchers pool their expertise to study the acoustic reflexes of the middle and inner ears. These major feedback pathways can have a variety of functions, one of which may be to improve signal detection in noisy environments. Understanding the operating characteristics of these reflexes is relevant to problems of the hearing impaired that are more complex than simple threshold changes. Another interconnected grouping of three projects is directed at how auditory information is processed by the central nervous system. From a systems perspective, two projects are proposed in which the neuronal processing schemes underlying pitch perception and the localization of sounds in space will be studied. The last project aims to measure and mathematically model the contribution of different neuronal types to the brainstem auditory evoked responses. The resulting information should eventually lead to new clinical uses of evoked potentials in the diagnosis of auditory malfunctions that have as yet no clearly articulated organic origins.